Conventional carburetors for internal fuel combustion engines are known to have a fuel-and-air mixing passage for delivering a controlled ratio of fuel-and-air to the combustion chamber of a running two or four stroke engine. The mixing passage is carried by a body of the carburetor and has a venturi disposed between an upstream region and a downstream region of the passage. Generally controlling or limiting the amount of air flowing through the venturi is a choke valve of a butterfly-type disposed within the upstream region of the passage. Generally controlling the amount of fuel-and-air mixture fed to the combustion chamber of a running engine is a throttle valve, also of a butterfly-type, which is disposed within the downstream region of the passage. As the throttle valve rotates from a substantially closed or slow idle position to a wide open throttle position and the choke valve is open, the engine rpm will generally increase from idle to maximum or full power. At wide open throttle, a vacuum induced at the venturi increases with the increased air flow demand of the engine. This causes an increase in fuel flow typically from a near atmospheric fuel supply chamber, through a fuel feed passage and a fuel orifice disposed at a radially most inward portion of the venturi.
The ratio of fuel-to-air of a running engine is generally less than the ratio necessary to reliably start a cold engine. The choke valve is primarily necessary to adjust the fuel-to-air ratio by controlling the air flow rate through the upstream region of the mixing passage. Prior to starting of a cold engine, the user must first manually place the choke valve in a substantially closed or “choke-on” position. The air flow is thus limited and a rich mixture of fuel-and-air flows through an intake manifold and to the combustion chamber of the engine via the pulsating vacuum induced by the reciprocating piston(s) of the engine.
Some typical carburetors are known to have a throttle follower arm which interacts with a latch or cam member of the choke valve actuation mechanism when the choke valve is in a closed position. This interaction is designed to slightly open the throttle valve from the slow idle position to a fast idle position for improved starting of a cold engine. The follower arm projects radially outward from a rotating shaft of the throttle valve and a coiled spring, engaged between the body and shaft or follower arm biases the throttle valve toward the slow idle position. When the choke valve is in the full open or off position, the throttle valve is free to rotate into the slow idle position via the bias of the throttle spring unless the user of the engine demands more power and manual rotates the throttle shaft against the resilience or biasing force of the throttle spring toward the wide open throttle position via some conventional throttle lever or mechanism which interacts directly with the user.
During cold starting of an engine, the choke valve is typically rotated manually away from the open position causing a distal end of the cam member, which projects radially outward from a choke shaft, to engage an opposing concave edge of the follower arm. This engagement causes the throttle valve to automatically rotate from the spring biased slow idle position to a preestablished fast idle position that will deliver more fuel and air to the combustion chamber than the slow idle position during starting. The opposing concave edge has a constant radius measured with respect to the rotational axis of the choke valve and which corresponds to the length of the cam member from the same axis so that regardless of the choke valve position, (other than the substantially open position) the throttle valve remains in the fast idle position during starting.
Unfortunately, once the engine has started, the user must remember to manually place the choke valve in the open or “choke-off” position to lean-out the fuel-and-air mixture to achieve smooth running of the engine. If the user does not timely remember to manually place the choke valve in an open or “choke-off” position after start-up, and during initial running conditions, the engine may stall on an overly rich mixture of fuel-and-air, or, a black smoke will be emitted from the exhaust, indicative of an unwanted increase in hydrocarbon emissions. Moreover, if the user attempts to increase rpm's of the running engine with the choke valve substantially closed, the air demands of the engine will not be met and the engine will stall on an excessively rich mixture of fuel-and-air. This condition aggravates manufacturer compliance with various regulatory emission standards established by governmental agencies. Moreover, for small displacement engine applications utilizing manual pull start cords, the engine may not start on the first pull. Unfortunately, a second pull of the pull start cord with the choke valve fully closed and the throttle valve in the fast idle position may flood the engine without an increase in air flow for the second start attempt.